In modern medical practice, it is common to implant radially expandable grafts and similar devices within body lumens (e.g., blood vessels, esophagus, common bile duct, ureter, urethra, fallopian tube, etc.)
In general intraluminal grafts and their respective support and/or attachment means fall into two major categories, self-expanding and pressure expandable. Self-expanding intraluminal grafts, are formed of resilient or shape-memory material such as spring steel or Nitinol.TM.. Self-expanding material is capable of being formed in a configuration from which it may be compressed to a radially compact diameter for placement within a damaged vessel. At the time of use, the memory feature of these materials causes them to self-expand from the radially compact diameter to the expanded operative diameter.
Pressure-expandable intraluminal grafts are formed of plastically deformable material such as stainless steel that is initially formed in its radially compact diameter. This type of material does not have memory, and will remain in the radially compact diameter until manually expanded. Typically, outwardly directed pressure is exerted upon the graft through use of a balloon so as to cause radial expansion and resultant plastic deformation of the material to its operative diameter.
The prior art has included numerous endovascular grafts of varying design. In general, these endovascular grafts typically comprise: a tube of pliable material (e.g., expanded polytetrafluoroethylene (ePTFE) or woven polyester) in combination with a graft anchoring component (e.g., a wireform, a frame, a series of wire rings, hooks, barbs, clips, staples, etc.) which operates to hold the tubular graft in its intended position within the blood vessel. Most commonly, the graft anchoring component is formed of a radially expandable frame (e.g., one or more radially-expandable wireforms of the type described hereabove) which is either a) incorporated into the body of the tubular graft or b) formed separately from the graft and positioned within the graft lumen, such frame being expandable to exert outwardly directed radial pressure against the surrounding blood vessel wall--thereby frictionally holding the graft in place. In operation, endovascular grafts which incorporate radially expandable graft anchoring devices are initially disposed in a radially collapsed configuration which is sufficiently compact to allow the graft to be transluminally advanced through the vasculature until it reaches the intended site of implantation. Thereafter, the graft (and the accompanying graft anchoring device) expands to a radially expanded configuration which is large enough to exert the desired outwardly-directed pressure against the blood vessel wall.
In some embodiments, hooks, barbs, or other projections formed on the graft anchoring device, will insert into the wall of the blood vessel to ensure that the graft will be firmly held in its desired position, without slipping or migrating after implantation. Like the above-described wireforms, the radially expandable anchoring devices (e.g., frames or wireforms) of endoluminal grafts are generally classifiable as either a.) self-expanding or b) pressure-expandable. Graft anchoring devices of the self-expanding type are usually formed of a resilient material (e.g., spring metal) or shape memory alloy which automatically expands from a radially collapsed configuration to a radially expanded configuration, when relieved of surrounding constraint (e.g., a surrounding tubular sheath or catheter wall). On the other hand, those of the pressure-expandable variety are typically formed of malleable wire or other plastically deformable material which will deform to a radially expanded configuration in response to the exertion of outwardly directed pressure thereagainst--as by inflation of a balloon or actuation of another pressure-exerting apparatus which has been positioned within the graft anchoring device.
In order to facilitate the transluminal, catheter based implantation of endoluminal grafts, it is typically desirable to minimize the diameter of the graft while in its radially compact configuration, thereby minimizing the required diameter of the delivery catheter used for delivery and implantation of the graft. In general, the smaller the diameter of the delivery catheter the smaller the size of the percutaneous puncture tract required for introduction of the catheter. Moreover, in many applications, it is necessary for the catheter to fit through relatively small body lumens and, thus, the diameter of the catheter must not exceed that of the body lumen(s) through which it must pass.
Moreover, the act of radially compressing or collapsing a graft can result in the introduction of stress within the wireform. In some cases, the introduction of such stresses can adversely affect the performance of the graft following implantation.
In view of these considerations, there exists a need in the art for the development of new wireforms which i) are capable of being radially compressed or radially collapsed to a small diameter so as to be useable in conjunction with relatively small diameter delivery catheters, and ii) will undergo minimal stress inducement during the radial compression/compaction process so as to minimize any deleterious effects that induced or residual stress may have on the subsequent performance and useful life of the graft.